At present, a mercury lamp is mainly used as a light source used for a projection display apparatus such as a projector. However, in order to prevent environmental pollution, there is a demand for development of mercury-free products. In order to address this situation, attention is now focused on a projector that uses a light-emitting diode (LED) or a laser diode (LD) not including any mercury (as discussed in JP2012-8549A). The LED or the LD is known as a light source configured to emit monochromatic light. In particular, a laser beam has high directivity, monochromaticity, and coherency, and thus can realize a projector having high luminance and high color reproducibility.
The “monochromatic color” is not limited to light having only a single wavelength, but includes light having a wavelength distribution of a level that enables a person to recognize a single color. In other words, in the Description, the “monochromatic color” includes light having a certain level of a wavelength distribution around a peak wavelength.
The laser beam is coherent light, and consequently direct viewing of the laser beam may harm a human body even if the laser output is low. This necessitates consideration being given to the safety of the human body. However, there is a trade-off relationship between safety and brightness. In other words, when the output of a laser light source is reduced, taking safety into consideration, brightness is reduced. Conversely, when brightness is increased, the output of the laser light source increases and this results in a decrease in safety.
Therefore, there is a desire that an illumination optical system capable of emitting monochromatic light having high luminance while taking safety into consideration be provided.